Recent advances in the design of superconductors have allowed the production of vastly improved high critical current density superconductors. For example, the Re--Ba--Cu--O (abbreviated ReBCO; Re denotes a rare earth metal or Y) family of superconductors have much higher intrinsic limits and can be used in high magnetic fields at relatively high temperatures (77K). These superconductors have been shown to possess high critical current densities (J.sub.c) at high temperatures when fabricated as single crystals or in essentially single-crystal form as epitaxial films on single crystal substrates such as SrTiO.sub.3 and LaAlO.sub.3. However, these superconductors have proven intractable to existing ceramics and materials processing techniques to the formation of long lengths of conductor having J.sub.c 's comparable to epitaxial films.
It has been demonstrated that in ReBCO superconductors, a biaxial texture is required to obtain high transport critical current densities. Texturing may be achieved by deposition of the ReBCO film on a biaxially textured, non-superconducting oxide buffer layer which is first deposited using ion-beam assisted deposition (IBAD) or inclined substrate deposition (ISD). The IBAD and ISD both tend to be slow, expensive processes that are not amenable for scale up to produce long lengths required for many applications. A method involving the formation of superconductor film on a biaxially textured metallic substrate is disclosed in U.S. patent application Ser. No. 08/419,583. This method is more amenable to producing long lengths of superconductors, which have applications in the development of superconducting current leads, windings for high-field magnets and motors, transmission lines, and other electrical conductor applications.
YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO) is an important superconductor for the development of articles requiring long lengths of superconductors. One method for manufacturing a superconducting wire or tape involves the deposition of a YBCO film on a metallic substrate. Superconducting YBCO has been deposited on polycrystalline metals in which the YBCO is c-axis oriented, but not aligned in-plane. However, in order to carry high electrical currents and remain superconducting, the YBCO films must be biaxially textured, preferably c-axis oriented, with effectively no large-angle grain boundaries, which reduce the current carrying capability of the material. YBCO films deposited on polycrystalline metal substrates do not generally meet these criteria.
The epitaxial growth of a YBCO film on a substrate may be achieved by a carefully controlled, single step process requiring the simultaneous application, in the presence of oxygen, of all three metal constituents (Y, Ba, and Cu) to a heated substrate. Another method for forming a YBCO film is known as the BaF.sub.2 method (Mankiewich, et. al Appl. Phys. Lett 51:1753-1755, 1987). This method is a two step process involving a deposition step and an annealing step. In a deposition step, the Y, Cu, and Ba (supplied as BaF.sub.2) are applied to a substrate, which need not be heated, to form a precursor layer. The rates of deposition of each constituent need not be carefully controlled and oxygen need not be supplied. Following deposition, the epitaxial YBCO structure is formed by annealing at temperatures of about 650-850.degree. C. This method has been successfully employed only in combination with certain specifically matched, single crystal substrates, e.g. SrTiO.sub.3. However, these materials are not suitable for the manufacture of long lengths of superconductors. What is needed in the art is a method for making high critical current density YBa.sub.2 Cu.sub.3 O.sub.7 superconducting layers on metallic substrates.